Optical communication systems are a substantial and fast growing constituent of communication networks. Wavelength Division Multiplexing (WDM) (and its more recent incarnation, Dense WDM or DWDM) is one technique used to increase the capacity of optical transmission systems. A wavelength division multiplexed optical transmission system employs plural optical channels, each channel being assigned a particular channel wavelength. In a WDM system, optical channels are generated, multiplexed to form an optical signal comprised of the individual optical channels, transmitted over a waveguide, and demultiplexed such that each channel wavelength is individually routed to a designated receiver.
WDM systems have been deployed in long distance networks in a point-to-point configuration consisting of end terminals spaced from each other by one or more segments of optical fiber. Because the communication traffic in such systems commonly travels many hundreds of kilometers, the need for add-drop multiplexing of individual channels is infrequent, occurring at widely spaced add-drop nodes. In metropolitan areas, however, WDM systems having a ring or loop configuration are currently being developed. Such systems typically include a plurality of nodes located along the ring. At least one optical add/drop element, associated with each node, is typically connected to the ring with optical connectors. The optical add/drop element permits both addition and extraction of channels to and from the ring. A particular node that allows the addition and extraction of all the channels is commonly referred to as a hub or central office node, and typically has a plurality of associated add/drop elements for transmitting and receiving a corresponding plurality of channels to/from other nodes along the ring. Metropolitan communications systems clearly require considerably more extensive add-drop multiplexing in order to successfully implement wavelength division multiplexing in their short-range systems.
Optical filters are essential components of add-drop multiplexers to select certain wavelengths and reject others. These filters need to have sharp passbands, i.e., passbands in which the transmission changes very rapidly with wavelength, to prevent crosstalk between adjacent channels. Moreover, when two or more filters are cascaded, the overall passband is smaller than the passband of the individual filters. That is, the overall bandwidth narrows due to the cascading of the filters. As a result it becomes even more important to employ sharp filters when WDM signals are required to traverse a large number of add-drop multiplexers such as in a metropolitan communication system. This requirement becomes more stringent as the spacing between adjacent wavelengths continues to decrease. In current WDM systems, for example, adjacent wavelengths may only be separated by 1 nm or less. It is generally recognized that when a wavelength is dropped at a single location in current WDM systems it should be isolated to about −30 dB with very low extinction at adjacent wavelengths. This requirement arises from the need to limit multi-path interference (MPI), also referred to as in-band crosstalk, which causes severe signal degradation when a residual signal at the same wavelength coherently interferes with a DWDM channel. It should be noted that this crosstalk arises from optical channel processing components within the node rather than the traditional source of MPI, which arises within the transmission span from two discrete reflections.
Such a coherent interference scales as the square of the intensity of the crosstalk signal for a single interference such as imperfect isolation of a single dropped signal described above. While this is a very demanding requirement, a much worse case arises in a transparent optical network when one or more spurious paths of a signal arise from the optical signal processing within a node. When this occurs, a single optical signal routed through many nodes can experience small coherent crosstalk contributions in each node. If the pathlength differences experienced by these crosstalk sources are similar, the worst case crosstalk contributions also sum coherently as the square of individual intensities when polarization and phase align to allow constructive interference of the interference signals.
Assuming the existence of n independent interference paths of equal intensity within the path of a signal through the network, and that at some period of time all n sources of the in-band crosstalk will constructively interfere, it can be shown that the maximum crosstalk value tolerable for those n crosstalk sources is less than the single node value of −30 dB by 20*log(n). Thus, for a signal that traverses N nodes and experiences n sources of MPI in each node, the maximum coherent crosstalk allowable at each node is −30 dB−20*log(N*n), which corresponds to <−54 dB for a signal with one MPI path per node traversing 16 nodes.
The worst-case situation described above will arise in practice within a conventional unidirectional WDM optical system if wavelength channels are demultiplexed for individual channel optical Add/Drop, and then multiplexed back together. This arrangement allows only one low loss path for a given wavelength through the device, however a small amount of each channel will transmit as crosstalk through the low loss paths of other channels. The highest crosstalk through such a path is typically an adjacent channel, where a worst case crosstalk for a demultiplexer and multiplexer are −25 dB and −10 dB respectively. Thus a net crosstalk of −35 dB can transmit through two paths (adjacent channels on either side), which is larger than the total maximum coherent crosstalk specified in the equation above for one node with two crosstalk paths (−36 dB). Lower levels of crosstalk in this application can be realized at the expense of added filter narrowing, cost and insertion loss by replacing the multiplexer device with a demultiplexer. This would yield a worst case crosstalk through the node of −50 dB, which for two paths at each node (n=2), would only be sufficient in the above equation for N=5 nodes.
The above requirements highlight the need for filters with high adjacent channel crosstalk in metropolitan area systems because of their extensive use of add/drop multiplexers to add and drop wavelengths at virtually every node, which means that each node is a potential source of coherent interference, or additional MPI. It can be demonstrated, for example, that for a metropolitan area system having 16 nodes with a maximum degree of flexibility in which every wavelength can be accessed at every node, requires the ability to filter or isolate individual wavelengths to about −55 dB or more in each node
Unfortunately, the vast majority of filters employed in communications systems exhibit a larger wavelength-dependent phase disturbance or group delay (chromatic dispersion) as the filter passband becomes sharper to reduce adjacent channel crosstalk. Dispersion limits the maximum distance a signal can travel before undergoing an unacceptable degree of degradation. While in principle it is possible to have a filter with a perfectly square passband, it comes at the expense of an unacceptably large amount of dispersion. Thus there is a tradeoff that needs to be optimized between the sharpness of a filter's passband and its dispersion. While current filter technologies can largely achieve the −30 dB of isolation required for a single MPI source, it becomes much more problematic to use current channel spacings and achieve 55 dB of isolation while maintaining dispersion at acceptable levels. Moreover, in future communication systems where each channel is expected to transmit at higher bit rates at narrower channel spacing, the limitation imposed by dispersion becomes even more severe.
Accordingly, there is a need for an arrangement that provides sufficient filtering for use in metropolitan area WDM communication systems without increasing the dispersion by an undue amount.